If I'm fighting Mutons, Heavy Floaters, or Cyberdiscs, they will tend to eschew their primary attack in favor of hucking a grenade in my general direction if my squad is spaced together too tight.

I know that there's a non-zero chance that they will just grenade one of my squad members anyway, but is there a good rule of thumb for how far apart my squad members should be in order to avoid a guaranteed grenade? I assume it must be related to the radius of the grenade's blast, but I rarely use grenades myself so I don't have this info committed to memory already.

2 Answers
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Visually, it looks like the effect radius of a grenade is a little larger than 2.5 tiles. It easily encompasses 5 tiles in a row (so if your soldiers are standing on tile #1 and tile #5 - 3 tiles in between - a grenade will easily include both soldiers' tiles completely. If your soldiers are standing on tile #1 and tile #6 - 4 tiles in between - it looks like it is still not that hard to hit them both with the grenade (but it's right at that edge where if you were trying, you'd be spending some time fiddling with the exact right placement). Soldiers at #1 and #7 (5 tiles in between) are safe.

I tested this by placing two soldiers 3,4, and 5 squares apart and seeing if I could fit the grenade indicator in between them without the "friendly fire" indicator (blue grenade indicator turns orange) turning on.

Alien grenades have the same AoE radius as regular ones (but better damage); if two of your people are close enough together that a single grenade could blast them both, chances are a muton or floater that manages to get into chucking range will toss one -- especially if they're both behind cover and its regular hit chance would be low.

The AI is reasonably clever; ask yourself "If I saw two enemies this close together, and behind cover where I'd have a poor chance of hitting them with regular fire, would I chuck a grenade?" If the answer is yes, then assume that the AI will do so if given the chance.

Also bear in mind that the grenade blast will injure your people and strip their cover and give any other alien units in the area a free shot. The AI will factor that into its consideration as well -- and remember that it isn't interested in keeping as much of your equipment intact as possible.